1. a) Field of the Invention
The present invention relates to an indium tin oxide (ITO) film contact structure, a thin film transistor (TFT) substrate, and its manufacture, and more particularly to a structure for electrically contacting an ITO film with an Al alloy film, a TFT substrate having such a contact structure, a method of manufacturing such a TFT substrate, and a TFT substrate with external connection terminals.
2. b) Description of the Related Art
FIG. 8 is a cross sectional view of a TFT and a pixel electrode of a conventional active matrix type liquid crystal display panel. TFT's are disposed in a matrix pattern on an image display area of a glass substrate 1, and external terminals 3 are disposed in a border area around the image display area. FIG. 8 shows a single TFT 10 among a plurality of TFT's . TFT 10 is constituted of a gate electrode 11, a channel layer 12, a channel protective film 18, a source electrode 13S, and a drain electrode 13D. The gate electrode 11 is disposed on the surface of the glass substrate 1. A first insulating film 4 is formed on the glass substrate 1, covering the gate electrode 11, and the channel layer 12 is formed on the first insulating film 4, overriding the gate electrode 11. The channel protective film 18 protects a surface of the channel layer 12 over the gate electrode 11.
Partial surfaces of the channel layer 12 on both sides of the gate electrode 11 are covered with the source electrode 13S and drain electrode 13D. Each of the source electrode 13S and drain electrode 13D has a four-layer structure having an amorphous silicon film 14, a lower Ti film 15, an Al film 16, and an upper Ti film 17 laminated in this order from the bottom.
A second insulating film 30 is formed on the first insulating film 4, and covers TFT 10. An opening 31 is formed through the second insulating film 30 in an area corresponding to the source electrode 13S. An indium tin oxide (ITO) film 35 is formed on the inner surface of the opening 31 and on a partial surface of the second insulating film 30. The ITO film 35 is connected to the source electrode 13S at the bottom of the opening 31.
In the border area, the external terminal 3 is covered with the first and second insulating films 4 and 30. An opening 40 is formed through these two first and second insulating films 4 and 30, the opening 40 exposing a partial top surface of the external terminal 3.
The lower Ti film 15 inserted between the Al film 16 and amorphous silicon layer 14 prevents the element performance from being degraded by Al diffusion. If the Al film 16 is directly contacted with the ITO film 35, a contact resistance is high. The upper Ti film 17 inserted between the Al film 16 and ITO film 35 lowers the contact resistance.
In the conventional active matrix type liquid crystal display panel shown in FIG. 8, the openings 31 and 40 are formed at the same time. The depth of the opening 31 corresponds to the thickness of the second insulating film 30, whereas the depth of the opening 40 corresponds to a total thickness of the first and second insulating films 4 and 30. Therefore, while the first insulating film 4 is etched to form the opening 40, the upper Ti film 17 on the bottom of the opening 31 is exposed to the etching atmosphere. If the upper Ti film 17 on the bottom of the opening 31 is completely removed, the ITO film 35 directly contacts the Al film so that the contact resistance becomes high.
In order to leave the upper Ti film 17 on the bottom of the opening 31 with good reproductivity, it is necessary to make the upper Ti film 17 sufficiently thick. For example, it is preferable to make the upper Ti film 17 have a thickness of 100 nm or more. As the upper Ti film 17 is made thick, it takes a longer time to etch and pattern the source and drain electrodes 13S and 13D, so that improvement on productivity is hindered.
FIG. 9 is a cross sectional view of a terminal formed on the surface of a conventional TFT substrate. A gate insulating film 4 covers the surface of a glass substrate 1. An amorphous silicon film 12a is formed on a partial surface area of the gate insulating film 4. A terminal 21a is formed on the amorphous silicon film 12a. The terminal 2 la has a lamination structure of an amorphous silicon film 14a, a lower Ti film 15a, an Al film 16a, and an upper Ti film 17a, sequentially laminated in this order. In the pixel area, the amorphous silicon film 12a constitutes a channel layer of a TFT, and the four layers from the amorphous silicon film 14a to the upper Ti film 17a constitute a source electrode, a drain electrode, and a drain bus line, respectively of TFT.
A second insulating film (protective insulating film) 30 is formed on the first insulating film (gate insulating film) 4, covering the lamination structure from the amorphous silicon film 12a to the upper Ti film 17a. A contact hole 32 is formed through the protective insulating film 30, in an area above the terminal 21a. A terminal protective conductive film 35a made of indium tin oxide (ITO) covers the inner surface of the contact hole 32 and the nearby surface of the protective insulating film 30. The terminal protective conductive film 35a prevents corrosion and damages of the terminal 21a. The terminal protective conductive film 35a is formed at the same time when a pixel electrode is formed in the pixel area.
A probe is made in contact with the surface of the terminal protective conductive film 35a to conduct a conduction test and an insulation test. A terminal for a tape automatic bonding (TAB) terminal has a similar structure to that shown in FIG. 9.
In the past, there was a case that when a probe was made in contact with the surface of the terminal protective conductive film 35a shown in FIG. 9, the probe broke through the terminal protective conductive film 35a and gave damages to the underlying upper Ti film 17a and Al film 16a.